Cable for railway vehicles has to show excellent properties such as thermal resistance and oil resistance and maintain the properties for a long time under various weather conditions such as high/low temperature and a sudden temperature change because it is used under extremely severe conditions, compared to general cables. Also, the vehicle should essentially have stability properties, for example flame retardance and low smoke, so as to secure safety of human life, cargo and equipment and to minimize losses if a fire breaks in the vehicles. A great interest has been increasingly taken in applications and roles of the cable for vehicles due to expanded construction of a high-speed railway and its increased use in recent years, therefore there is still required for a cable for vehicles showing more excellent properties.
Until now, a base resin including halogen, for example polyvinyl chloride, polychloroprene or polychlorinated polyethylene, was generally used as a covering material of the cable for railway vehicles. However, such a base resin including halogen has disadvantages of being difficult to obtain a flame-retardant covering material having less than 0.5% of halogen and less than 1.5 of toxicity index, and also of having a poor thermal property at high temperature.
To solve the above-mentioned problems, a large quantity of metal hydroxide might be used as a halogen-free flame retardant to obtain a cable satisfying standard levels on halogen content and flame retardance, as well as low smoke and low toxicity index. But, the cable has problems of having extremely deteriorated properties such as a mechanical property and properties after heating at high temperature due to such a large quantity of the flame retardant, and also having a deteriorated oil resistance against oils such gasoline or polar oils.
Meanwhile, a polar resin having high content of a polar group, a crystalline resin having a melting point higher than that in a test condition on an oil resistance, or an engineering plastic was used as a basic resin so as to satisfy the oil resistance in the prior art. However, the resin, which has a high melting temperature and a crystalline structure, and the polyester-based engineering plastic have problems of being difficult to install them in a narrow space and a complex structure, and showing a poor bending durability due to a very low flexibility for repeated movements and bendings even after their installations, because they show a remarkably high solidity and a greatly low flexibility at room temperature. In addition, it is not possible to use such a soft and flexible material in specific sections subjected to stress and pressure because the material is easily deformed due to its greatly low resistance against dynamic pressure at high/room temperatures if a rubber material with a polar group having an excellent resistance against specific oils is used for the purpose of improving the oil resistance.
As described above, a cable and a cable-covering material, which have stabilities upon the firing and also show properties suitable to the environment and a high durability upon its use, continue to be on demand in recent years.